The present invention relates to a circuit for complementing the supply voltage, particularly for resistive loads. More particularly, the present invention relates to a circuit for complementing the supply voltage that is suitable to be used ahead of a light source such as an incandescent lamp.
It is commonly known that the best light source currently available is constituted by filament lamps, which in optimum conditions (for example a 220 V supply voltage) constitute the cheapest commercially available light sources and offer the best chromatic yield (RA=100).
However, these lamps, which are supplied with a sinusoidal AC voltage at the frequency of 50 or 60 Hz (standard values of the mains), are subject to drawbacks in their use, since they are very negatively affected by any voltage decreases.
It is in fact known that the mains voltage (which is 220 V in Europe and is the rated value of the mains voltage for household use) is rarely usable, since depending on the distance from the distribution points there are voltage decreases due to voltage losses on the lines and to similar factors. In this way, the lamp designed to operate with a rated voltage that is equal to the ideal mains voltage (for example 220 V) is actually often supplied with a distinctly lower voltage.
This drawback is strongly felt in technologically backward countries, where the mains voltage can be affected by decreases, reaching approximately 170 V at the points of use, with a consequent high loss of efficiency equal to approximately 4.5 lumen/watt.
In this case, an incandescent lamp is not used at its best and the amount and quality of the light produced are considerably lower than the potential of the lamp.
Experimentation has in fact shown that in incandescent lamps with a tungsten filament, said filament is designed to operate at the rated mains voltage, but this occurs only for very short periods; accordingly, the frequencies emitted as photons tend to shift, within the spectrum, more towards the infrared than towards the ultraviolet. In this manner, the incandescent lamp used in a known manner uses part of the consumed energy to produce radiation in a range that is beyond the visual spectrum of the human eye, thus producing heat to the detriment of luminous efficiency.
The luminous efficiency of an incandescent lamp, for example a 40-watt lamp, in optimum operating conditions (for example 220 V AC), is approximately 8.5 lumen/watt. As the voltage decreases, for example to 210 V, the efficiency drops to approximately 7.35 lumen/watt. In practice, a loss of approximately 1 lumen/watt for every 10 V of mains voltage decrease is observed.
The decrease in efficiency caused by the decrease in the supply voltage is common to all devices supplied with AC power. For example, in the case of a water heater designed to operate at a preset operating voltage so as to bring water to the boiling temperature (approximately 100.degree. C.), a lower-than-rated supply voltage can decrease the efficiency of the heater to the point of preventing it from reaching the boiling point of water, thus no longer performing the intended task.
Going back to incandescent lamps, in order to fully exploit these lamps it would be necessary to use a transformer or a voltage booster so as to raise the mains voltage to the rated value required by the lamp. The drawbacks of this solution are the cost, bulk, and consumption of these devices, as well as the difficulty in using them in a household environment.
Another drawback of conventional types of incandescent lamps is the high peak current that they must withstand at power-on time. If the lamp is switched on at the peak point of the sinusoidal curve of the AC voltage, and assuming the presence of an optimum rated voltage level (for example 220 V), the current that flows through the filament of the lamp is very high with respect to the current that flows when the lamp is already switched on.
Therefore, assuming that one is at the peak, at 220 V, the effective voltage is much higher and is precisely 220.sqroot.2, that is to say, approximately 310.2 V. At this voltage, since an incandescent lamp offers very low resistance when cold, a current of approximately 7-10 A flows through the lamp, causing magnetization of the filament and a violent contraction thereof, with breakage and projection of the pieces of filament.
Solutions have been proposed which supply an incandescent lamp with a DC current, which as known cannot be drawn directly from the electric mains.
For this purpose, it has been proposed to use a conventional current rectifier (for example of the diode-bridge type to achieve full current rectification) arranged ahead of the load, with at least one smoothing capacitor parallel-connected thereto to obtain a perfectly DC output current to supply the load.
Such a configuration is known, for example, from British patent application no. 2,051,505. In this document, the above described rectifier circuit is used ahead of an inverter to which an incandescent lamp is parallel-connected.
The purpose of the circuit proposed by the British document is to allow to supply a lamp equally with AC current (in normal operating conditions) or with DC current (in emergency conditions), making the inverter operate correctly in both of the above described operating conditions.
Another example of application of a rectifying and smoothing circuit ahead of the load can be found in European patent application no. 0 254 506. In this second document, the capacitive means arranged after the rectifier have the purpose of limiting and smoothing the voltage peaks that are normally present in the mains AC voltage, in order to avoid excessive voltages on the load that is being supplied.
The capacitive means therefore have the purpose of protecting the load, which is supplied with a DC voltage.
Both of the above described prior documents use the rectifying and smoothing circuit to obtain a perfectly DC output current (voltage). For this purpose, the smoothing capacitor or capacitors have sizes that are selected so as to "clip" the voltage peaks and fill the "troughs" of the rectified voltage. This effect is shown in FIG. 2.
The value of the smoothing capacitor must therefore necessarily be chosen large enough to perfectly level the peaks of the rectified mains voltage, preventively reduced by the .sqroot.2 factor. That is, for example, 220/.sqroot.2=156V which, transformed in direct voltage will be equal to 156.times..sqroot.2=220V(DC).
The drawback of DC power supply is the fact that the incandescent lamp considerably reduces its life and that the high-value capacitor is difficult to integrate or in any case insert in a conventional light bulb or in the lamp holder.